With a seamer for doubly seaming a can end to a can barrel, the can end is fitted on the can barrel, then the can barrel and can end are raised by a lifter to secure the can end in a state pressed against a seaming chuck, and a seaming roll moving around the can while it is rotating is gradually brought closer to a flange of the can barrel and curl portion of the can end, thereby forming a double seam. In this operation, if the lifter pressure with which the can barrel and can end are raised by the lifter is insufficient, overlapped portions of the can barrel and can end at the double seamed portion have an insufficient length, so that the finished can is prone to leakage. If the lifter pressure is excessive, on the other hand, the overlapped portion of the can end has an insufficient length, thus resulting in a double seam in which the end of the can barrel abuts the can end. In this case, not only is the finished can prone to leakage, but also the can end is liable to be cracked in a portion where the end of the can barrel abuts the can end.
Therefore, it is necessary to control the lifter pressure to a proper value at all time. To this end, there is an apparatus of the hydraulic oil type for measuring the lifter pressure.
This hydraulic oil apparatus for the lifter pressure measurement comprises a vessel assembled in a body of the apparatus, expandable and contractive in the direction of pressure application, and filled with oil; a mechanism for causing expansion and contraction of the body in the direction of pressure application; a member for transmitting the pressure applied to the body to the vessel at all time; and a Bourdon pressure gauge connected by a tubeline to the vessel in the body. A pressure applied to the body is thus transmitted to the vessel to cause deformation of the vessel. In consequence, oil is caused to enter the pressure gauge, whereby the applied pressure is displayed on the gauge.
In a method of measuring the lifter pressure using this hydraulic oil lifter pressure measurement apparatus, the body is clamped between the lifter and chuck, and the body is elongated to a predetermined extent from a start point of measurment right before the pressure gauge pointer starts movement. The lifter pressure is measured from the difference of the reading at the start point of measurement of the pressure gauge and the reading after the expansion to the predetermined extent.
With the prior art hydraulic oil lifter pressure measurement apparatus, which has the above construction and adopts the above method of measurement, the values measured do not vary linearly on the basis of a spring constant but great errors result due to such causes as defective deformation or restoration of the vessel filled with oil, oil leakage from the vessel and tubelines, defective deformation of the elastically expandable and contractive portion of the Bourdon pressure gauge due to fatigue, changes in the oil volume with temperature changes and changes in the oil volume due to intrusion of air, etc.
Further, errors are liable to the reading of the pressure gauge pointer depending on the level of the eyes.